1. Field of the Invention
The present invention relates to a liquid crystal polyester-containing liquid composition, namely, a liquid composition which contains a liquid crystal polyester.
2. Description of the Related Art
Recently, various studies have been made on application of a liquid crystal polymer, especially a liquid crystal polyester, as a material constituting an insulating material (for example, an electronic substrate) because of having excellent high frequency characteristics and low hygroscopicity. It is possible to exemplify, as such an insulating material, a liquid crystal polyester-impregnated base material produced by a method in which a base material is impregnated with a liquid composition containing a liquid crystal polyester, which contains liquid crystal polyester and a solvent, and then the solvent is removed. In order to produce a liquid crystal polyester-impregnated base material having satisfactory quality, the liquid crystal polyester is desirably a liquid crystal polyester having satisfactory solubility in a solvent to be used.
For example, JP-A-2004-315678 (corresponding to CN101104705A and KR2004090717A) discloses that a liquid crystal polyester (aromatic liquid crystal polyester) including (1) a repeating unit derived from an aromatic hydroxycarboxylic acid, (2) a repeating unit derived from an aromatic dicarboxylic acid, and (3) a repeating unit derived from an aromatic amine selected from the group consisting of an aromatic diamine and an aromatic amine having a hydroxyl group is a liquid crystal polyester having satisfactory solubility in N-methylpyrrolidone. JP-A-2007-146139 (corresponding to CN1955253A and KR2007045095A) discloses that a liquid crystal polyester (aromatic liquid crystal polyester) with a limitation on a composition ratio of (1) a repeating unit derived from an aromatic hydroxycarboxylic acid, (2) a repeating unit derived from an aromatic dicarboxylic acid, and (3) a repeating unit derived from an aromatic amine selected from the group consisting of an aromatic diamine and an aromatic amine having a hydroxyl group, is a liquid crystal polyester which is soluble in N,N-dimethylacetamide as a solvent having a lower boiling point than that of the above-mentioned N-methylpyrrolidone, and also discloses that a liquid crystal polyester-impregnated base material can be easily produced by impregnating a base material such as an inorganic fiber or an organic fiber with a liquid composition containing a liquid crystal polyester, and then removing a solvent.
However, a conventional liquid composition containing a liquid crystal polyester left some room for further improvement with respect to stability of a viscosity and stability of a solubility during storage. In relation to the stability, the liquid composition containing a liquid crystal polyester, which contains a liquid crystal polyester and N-methylpyrrolidone, specifically disclosed in Examples of JP-A-2007-146139 (corresponding to CN1955253A and KR2007045095A) has such a problem that the liquid crystal polyester is precipitated, resulting in sedimentation as for stability of the solubility during storage at normal temperature, although the stability of the viscosity during storage is stable since it is hard to increase.
There has been known, as the method for the prevention of sedimentation of a resin in a liquid composition containing a resin, a method in which a sedimentation inhibitor is added to a liquid composition. For example, JP-A-60-231775 discloses that sedimentation of an unsaturated polyester can be suppressed by adding a sedimentation inhibitor to a mixture of an unsaturated polyester varnish and a white pigment. JP-A-7-266499 discloses that sedimentation of a thermosetting resin can be suppressed by adding fumed silica as a sedimentation inhibitor to a varnish containing a thermosetting resin and an inorganic filler, and thus producing a copper clad laminate in which inorganic fillers are uniformly distributed in a thickness direction.
However, an aspect of use of a sedimentation inhibitor for a liquid composition containing a resin had a problem that additional step for dispersing a sedimentation inhibitor in a liquid composition is required, and thus production costs of the liquid composition increase. There was also a problem that, even in case of performing dissolution of a resin in a solvent and dispersion of a sedimentation inhibitor in the solvent in one single step, filtration resistance increases in the filtration step for removing foreign substances in a liquid composition containing a resin and a sedimentation inhibitor, and thus it becomes hard to produce a liquid composition.